Various techniques have been employed to drill, for example, a pair of parallel wells 102, 104 for Steam Assisted Gravity Drainage (SAGD) applications, as shown in FIG. 1. An important objective when drilling such wells is to achieve proper placement of each well relative to each other. As used herein, the term “first” horizontal well is used to refer to well 102 to be first drilled (and in the prior art completed first), usually the lower producer well. In various embodiments, the “first” well 102 may be drilled slightly ahead of the second well. In contrast, the “second” well refers to the well 104 that is the upper well, completed second in the prior art.
Often a slant rig is employed to drill the shallow wells (a few hundred meters deep). With a slant rig, the drill pipe enters the ground at an angle of about 45°, so that the well can build quickly to 90°—i.e. horizontal. After being drilled in the desired zone, the first well 102 is completed with slotted liner and tubing. The slotted liner is typically 7 or 9⅝ inches in outer diameter (OD). The tubing is typically 3½ inches OD and extends to the toe of the well. A second tubing string might also be run to the heel of the producing well.
Referring now to FIG. 2, a wireline tool 202 is then deployed inside the tubing of well #1 102. The wireline tool 202 is requited to determine the distance between the two wells 102, 104 and their relative location, information needed to steer the second well (#2) parallel to well #1. The bottom hole assembly 212 (BHA) in well #2 104 includes an measurement-while-drilling (MWD) tool 214 and a directional system 216, such as a steerable motor with bent sub, or a rotary steerable system.
There are two well-known magnetic ranging techniques utilizing a wireline tool inside tubing.
In the first technique, the wireline tool 202 produces a magnetic field ({right arrow over (B)}) that can be measured by the tool 214 in well #2 (see U.S. Pat. Nos. 5,485,089, RE 36,569, “New Electromagnetic Ranging/Surveying Method for Drilling Parallel Horizontal Twin Wells”, A. Kuckes et al, SPE Drilling and Completion, June 1966, pages 85-90). The wireline tool 202 contains a large solenoid that produces a magnetic field with known strength and known field pattern. The tubing and slotted casing affect the magnetic field, but their effects can be removed by calibrating the solenoid inside the same size tubing and casing on the surface. The magnitude of the measured magnetic field indicates the separation of the two wells 102, 104, and the direction of the magnetic field indicates their relative positions.
Referring now to FIG. 3, in the second technique, strong permanent magnets are mounted in a near-bit sub 312 in well #2 104 while the wireline tool 302 contains magnetometers (see U.S. Pat. No. 5,589,775, “Magnetic ranging Technologies For Drilling Steam Assisted Gravity Drainage Well Pairs and Unique Well geometries—A Comparison of Technologies”, T. L. Grills et al, SPE paper 79005, Nov. 4-7, 2002). The permanent magnets rotate with the drill bit, thus producing a rotating magnetic field. As the drill bit passes by the wireline magnetometers 302, the rotating magnets 312 produce an oscillating magnetic field inside the tubing. The distance between the wells 102, 104 is deduced from the variation in the magnetic field with measured depth of the drill bit. This approach has the drawback that the near-bit magnetic sub is located between the bent sub and the drill bit, reducing the steering capability of the system.
Other methods have been proposed, but are not favored in drilling SAGD wells.
The Single Wire Guidance™ System (see U.S. Pat. No. 5,074,365, “Collision Avoidance Using a Single Wire Magnetic Ranging Technique at Milne Point, Ak.”, C. R. Mallary et al, IADC/SPE paper 39389, Mar. 3-6, 1998) involves a wire 402 carrying a current (I) to the toe of well #1 102, where the wire 402 is grounded to the casing 404 (FIG. 4). Most of the current returns to the surface through the well casing 404 and tubing 406; however, a very small amount of current leaks into the formation 200 at each foot along its length. The leakage current varies from foot to foot depending on the properties of the casing, the cement, and the formation resistivity. In general, the return current on the casing and tubing can be written as I′(z) where z is the measured depth. The net current along well #1 102 is I-I′(z). The net current is small, variable, and not well known. The net current produces an azimuthal magnetic field around the wellbore given approximately by
      B    →    =                              μ          0                ⁡                  [                      I            -                                          I                ′                            ⁡                              (                z                )                                              ]                            2        ⁢                                  ⁢        π        ⁢                                  ⁢        r              ⁢          n      ^        ×          r      ^      where {right arrow over (r)} is the radial vector from the wire to the observation point, r=|{right arrow over (r)}| is the magnitude of {right arrow over (r)}, {circumflex over (r)}={right arrow over (r)}/r is a unit vector that points from the wire to the observation point, {circumflex over (n)} is a unit vector that points along the axis of the wire, and μ0=4π·10−7 Henry/m is the permeability of vacuum. This magnetic field can be measured with three-axis magnetometers in the MWD tool 214 in well #2 104. The direction to the casing can be deduced from the three orthogonal components of the magnetic field. The distance to the cased well, however, is indeterminate without an accurate value for the leakage current versus depth, and there is no easy way to obtain I′(z).
A passive magnetic ranging technique involves inserting permanent magnets inside the steel casing. The permanent magnets are alternately magnetized N-S and S-N to create a discernable magnetic field pattern (U.S. Pat. No. 6,991,045). The magnetic field is measured by the MWD magnetometers, and the information employed to steer well #2. Afterwards, the permanent magnets must be recovered from the cased well.
The two standard magnetic ranging methods that require a wireline tool in the cased well are inefficient. Because the well is horizontal, the wireline tool must be pushed toward the well's toe as well #2 is progressively drilled. This requires a rig for well #1 just to move the wireline tool with drill pipe, or mud pumps to pump it down, or coiled tubing to push it down, or wireline tractor to pull it down. All of these methods are expensive and require additional equipment at the well site just to move the wireline tool.